The invention concerns a glass composition for use as sealing material in fuel cells! preferably in the solid oxide fuel cells (SOFC) of the stacked planar type. Typically, such fuel cells are composed of Y-stabilized ZrO2, (YSZ) electrolyte with electrodes and contact layers to the electron conducting plate Interconnect (IC), which makes the series connection between the cells. Gas tight sealings are vitally important for the performance, durability and safety operation of the fuel cells including the manifold and heat exchanger.
The difficulties in providing a suitable sealing material are numerous:
The sealing material should be able to adhere to the fuel cell components at a heat treatment not higher than 1300xc2x0 C. which is the maximum temperature heat treatment of a fuel cell stack, and be resilient in order to take up deformations, e.g. due to TEC differences between the fuel cell components, and at the same being able to withstand a certain overpressure at the operation temperature which require a viscosity of more than 105dPasxc2x7s. The thermal expansion coefficient (TEC) should be in the range 9-13xc2x710xe2x88x926Kxe2x88x921 in order not to initiate cracks in the fuel cell components. Furthermore, the sealing material has to be stable over a time span of say 40.000 h without deteriorating crystallization or reactions with the other materials as well as with the ambient gasses, atmosphere containing steam, methane, hydrogen, carbonmonoxide and carbondioxide or nitrogen and oxygen.
Glass or glass ceramic seals may fulfil the requirements established above and according to literature quite a range of potential glasses have been reported:
According to Ley et al. (1996), each of these glass types have a drawback: the alkalis and alkali silicates and borates will react with the fuel cell components. The alkali borate glasses have too low TEC and soda-lime glasses too low viscosity.
In contrast several glass compositions within the SrOxe2x80x94La2O3xe2x80x94Al2O3xe2x80x94B2O3xe2x80x94SiO2 system should be suitable (K. L. Ley, M. Krumpelt, R. Kumar, J. H. Meiser and I. Bloom, 1996, J. Mater. Res., Vol. 11, No 6, pages 1489-1493).
The present invention is in contrast to the conclusion of the authors above based on highly viscous polymerized alkali-alumina-silicate glass seals, which are reluctant to crystallize at elevated temperature. An example of a highly polymerized glass is pure SiO2 glass, which has a polymerized 3D network (as the crystalline phase, quartz) based on SiO44xe2x88x92 tetrahedra, where each oxygen ion connects two Si ions (B. E. Warren and Biscoe, 1938, J. Am. Ceram. Soc. 21, page 29). By addition of group I, II and III metal oxides this network is broken and the softening point, the viscosity and the melting point decreases significantly. It is possible to retain a polymerized structure of the melt with a high viscosity by substituting SiO2 with NaAlO2 (D. C. Boyd and D. A. Thompson, in Ullmann, Vol. 11, page 815). Accordingly, a NaAlSi3O8 melt has a high viscosity of 108.5 dPasxc2x7s at 1120xc2x0 C. (H. Rawson, 1967, Academic Press, London and New York, page 89). This melt is assumed to have a 3D network (Si1-x, Alx)O44xe2x88x92xxe2x88x92 network structure, where xNa+ compensate the extra negative charge, similar to the 3D network in the mineral albite with the same composition. Crystallization from such a highly viscous melt held nearly 100xc2x0 C. below the melting point may take years due to the high viscosity (H. Rawson, 1967). By addition or subtraction of NaAlSiO4, SiO2, it is possible to reach two eutectic melting temperatures at 1062xc2x0 and 1068xc2x0 at compositions: NaAlSiO4: SiO2, 37.0:63.0 wt % and 65.0:35 wt %, respectively (J. F. Schairer, J. Geol. 58, No 5, 514, 1950). For the system: KAlSiO2, SiO2, an eutectic point of 990xc2x120xc2x0 C. may be obtained at a composition of KAlSiO4: SiO2 equal to 32.8:67.2 wt % (J. F. Schairer, N. L. Bowen, Bull Soc, Geol. Finland, 20.74 (1947).
The TEC of a NaAlSi3O8 glass is 7.5xc2x710xe2x88x926Kxe2x88x921, which is lower than the SOFC components 10.0-13xc2x710xe2x88x926. The TEC of the albite glass can be increased slightly by addition of NaAlO2, whereas a value of 10.4xc2x710xe2x88x926Kxe2x88x921 may be obtained by addition of Na2O giving a cation composition of Na3.33Al1.67Si5 (O. V. Mazurin, M. V. Streltsina and T. P. Shvaikoshvaikoskaya, Handbook of glass data, part C, page 371, from K. Hunold and R. Brûckner, 1980a, Glastech. Ber. 53, 6, pages 149-161). Higher values up to more than 12xc3x9710xe2x88x926Kxe2x88x921 can be obtained by further addition of Na2O according to these authors.
An example of a NaAlSi3O8+Na2O TEC matched glass for sealing yttria stabilized zirconia is shown in FIG. 1. Addition of K2O will have an even higher effect on the TEC.
Addition of Na2O and K2O alone will decrease the viscosity and the Tglass and Tsoftening as illustrated in Table 2 for Na2O, which will be necessary for operation temperatures below 1000xc2x0 C.
Alkali-addition, however, will cause an increased reaction rate with the other fuel cell components and an evaporation of sodium and potassium, so that this solution is best suited for low operation temperatures. Small amounts of BO3 addition can also be used to decrease the melt temperature and viscosity. An alternative to the addition of alkalies in order to increase the TEC is to use fillers with a high TEC and (Y. Harufuji 1992: Japanese Patent No 480,077 A2) and (Y. Harufuji 1994, Japanese Patent No 623,784 A2) thus Harufuji mentions different fibres of carbon, boron, SiC, polytitanocarbosilane, ZrO2 and Al2O3 and powders of Al2O3, ZrO2, SiO2, MgO, Y2O3 and CaO and Al, Ag, Au and Pt. To this list we can add stabilized ZrO2, TiO2, MgOxe2x80x94MgAl2O4 composites, (Mg,Ca)SiO3, Mg2SiO4, MgSiO3, CaSiO3, CaZrO3 and MIIAlSi2O8, where MII=Ca, Sr and/or Ba (rare earth oxides, e.g. CeO2, Eu2O3 and ThO2) (Li2Si2O5 may be used at temperatures below 1000xc2x0 C.).
Other alkalisilicates may be used as fillers for low temperature operation. A combination of alkali and filler addition can be used to obtain optimal TEC, viscosity and the softening point Ts. Also addition of small amounts ( less than wt %) B2O3 instead of or together with Na2O combined with addition of high TEC fillers mentioned above is a possibility. The filler addition will reduce the exposed surface of the glass and thus the evaporation of the more volatile constituents of the glass.
Deteriorating Reactions May Involve:
(1) SiO evaporation may occur under reducing condition on the anode side condensation may take place in other areas of the fuel cell system. Apparently this process is slow.
(2) Volatile sodium and potassium may react with the other fuel cell materials, e.g. the chromite of the interconnection plate. The evaporation is strongly influenced by the sodium surplus of the glass. For this reason the sealing glasses with alkali/Al-ratios above 1 should only be used in fuel cells with low operation temperatures.
According to the invention there is provided a glass composition comprising a glass matrix with main components consisting of SiO2, Al2O3 and one or more compounds from group I metal oxides, and a filler material evenly dispersed in the matrix, wherein the filler material consists of particles of one or more refractive compounds from the group: (Al2O3, MgO,) rare earth oxides, MgOxe2x80x94MgAl2O4 composites, stabilized zirconia, (MgCa)SiO3, Mg2SiO4, MgSiO3 CaSiO3, CaZrO3, ThO2, TiO2 and the MIIAlSi2O8, where MII=Ca, Sr and/or Ba. For low temperature application alkalisilicate fillers may be used.
The filler material is added to the sealing glass in order to adjust the thermal expansion coefficient, so that it matches the TEC of the other parts of the fuel cell in addition the stability of the glass may be improved and the viscosity increased.
Preferred embodiments of the composition contain Na2O or K2O or both in amounts such that the stoichiometric molar ratio of Al2O2 to Na2O or K2O is in the range of 0.1-1.3 in order to reach an optimum TEC, while at the same time avoiding too much alkali metal that may react with the other materials in the cell stack.
One or more compounds of group II metal oxide may be components of the glass matrix. Known glass compositions with main components comprising SiO2, Al2O3, and one or more compounds from group I or group II metal oxides are advantageous for sealing fuel cells with gas separators of Laxe2x80x94Sr/Ca/Mgxe2x80x94Cr/Vxe2x80x94O interconnections of ceramic material or a metal alloy, e.g., Crxe2x80x94Fexe2x80x94Y2O3 material and with an operating temperature above 600xc2x0 C. In particular, compositions that within the glass have evenly dispersed a refractive filler material consisting of particles of one or more compounds from the group: MgO, MgOxe2x80x94MgAl2O4 composites, stabilized zirconia, rare earth oxides (especially Eu2O3 and CeO2), ThO2, TiO2, (MgCa)SiO3, Mg2SiO3, CaSiO3, CaZrO3, and MIIAlSi2O8, (MII=Ca, Sr or Ba).
In a preferred embodiment of the invention commercially available feldspar or nepheline syenite starting materials may be used for making the basic glass material.
The starting material is melted at about 1550xc2x0 C. for one hour in an alumina or platinum crucible. The melted material is then quenched in water, crushed and ground to glass powder with a particle size of less than 90 xcexcm. The glass powder having a TEC of about 75xc3x9710xe2x88x927/K is then mixed with a filler, e.g. MgO (TEC 130xc3x9710xe2x88x927/K) with a grain size of less than 10-40 xcexcm in ratio of 2:3.5 (vol.) in order to obtain a TEC of 110xc3x9710xe2x88x927/K.
Glass sealing are produced by filling the mixed powder into graphite forms followed by stamping and removal of excess of powder. The powder is then sintered in a furnace in N2-atmosphere at 750xc2x0 C. for 5 hours and at 1300xc2x0 C., for one hour. Glass seals for narrow gaps  less than 1 mm (e.g. in the electrode area) are produced by tape-casting of glass and filler mixtures.